Neuronal circuits in the auditory brainstem are specialized to extract and represent specific aspects of acoustic information. Inhibitory neurons feature prominently in these circuits, but how inhibition contributes to auditory processing is not well understood. The mammalian dorsal cochlear nucleus (DCN), one of the principal targets of auditory nerve fibers, is thought to process monaural cues used to localize sound sources. Feed-forward inhibition onto DCN principal cells from glycinergic neurons known as vertical cells is believed to contribute importantly to this function, yet the specific mechanisms that determine how activity in vertical cells is recruited and what effect these neurons have on their targets have not previously been identified. The objective of this proposal is to clarify the role of vertical cells in the DCN by directly investigating their cellular and synaptic properties. Whole-cell patch-clamp techniques, in combination with anatomical and immunohistochemical analysis, will be used to test the hypothesis that at least two functionally distinct subtypes of vertical cells account for the in vivo behavior of DCN principal neurons. To target recordings of vertical cells in a brainstem slice preparation, this study will take advantage of recently developed transgenic mice in which glycinergic neurons express green fluorescent protein. In Specific Aim 1, vertical cells will be categorized into different functional subtypes using electrophysiological and anatomical criteria. Specific Aim 2 will address the hypothesis that activity in vertical cell subclasses is differentially modulated by inhibition mediated by gamma-aminobutyric acid. Experiments in Specific Aim 3 will use dual whole-cell patch-clamp recordings between synaptically connected pairs of vertical cells and principal neurons to investigate whether subtype-specific inhibitory input from vertical cells contributes to the characteristic response properties of DCN principal cells. Successful completion of these aims will provide insight into the basic mechanisms underlying DCN function and may reveal previously unappreciated heterogeneity of vertical cell-mediated inhibition within the DCN. Public Health Relevance: Some forms of tinnitus (persistent ringing in the ears) are thought to arise from noise-induced or aging-related loss of vertical cell activity. Understanding the basic mechanisms underlying normal vertical cell function should provide insight into how loss of activity in these neurons may contribute to the pathophysiology of tinnitus and could suggest avenues for therapeutic intervention.